The present invention relates to a flow-through device for obtaining filtrate samples from pipelines which are used to transport slurries, such as fluid slurries in phosphoric acid plants.
The analysis of the composition of fluids at various stages during the production of phosphoric acid in commercial processes is essential for the effective control of product purity and for imporved operating efficiency. Continuous monitoring of product composition is particularly important when a relatively low grade of phosphate rock is available and the conservation of raw materials becomes essential. Various devices for analyzing the composition of process streams and for determining the specific gravity of process fluids, both continuous and manual, have been used in this endeavor.
In particular, accurate sampling of phosphoric acid at various stages of its manufacture is desired for chemical analysis and density determination, such as during the wet process manufacture of phosphoric acid from fluorapatite concentrate. In the wet process, finely ground phosphate rock is dissolved with sulfuric acid in an aqueous medium, precipitating gypsum. The dilute phosphoric acid formed helps to attack the phosphate rock particles, and the solution becomes supersaturated in calcium ions which are then precipitated with the sulfate ions present. The amount of calcium ions in solution is a function of the sulfate concentration, so that control of the sulfate content is necessary to control the crystallization. It is essential to maintain a small excess of sulfuric acid in the digestion slurry to obtain easily filterable and washable gypsum crystals. Separation of the gypsum crystals and the ability to wash them free of the digestion liquors is a determining factor on recovery of feed P.sub.2 O.sub.5 values in the acid product.
A typical procedure used in the plant for obtaining samples of phosphoric acid involves first purging several gallons of a slurry of phosphoric acid and calcium sulfate through the sample line. A ram valve is used to open and close the sample line. Then, approximately one liter of sample is obtained which is physically carried to a process control lab located nearby. The sample slurry is manually filtered using a laboratory funnel, and the filtrate is then manually analyzed for sulfate content using turbidometric methods.
This procedure is necessarily very cumbersome and time consuming. In addition, a relatively large amount of process slurry must be used to obtain a relatively small quantity of filtrate for analysis. Finally, sampling lines readily become plugged with deposits of solid material thus rendering automated sampling difficult if not impossible. In view of these difficulties, it would be highly advantageous to provide an automated sampling procedure which is relatively efficient in providing quality filtrate samples for analysis.
Various devices and methods have been previously proposed for improving the sampling procedure. It would be desirable to automate the sampling procedure and integrate the sampling apparatus with automatic analyzers.
U.S. Pat. No. 3,582,284 and 3,712,795, both to Hamshere et al., describe an automatic sampling method and device for obtaining samples of sulfate-containing phosphoric acid from plant process vessels such as chemical reactors. U.S. Pat. No. 3,966,606, to Ahmad, also describes a device for obtaining samples of phosphoric acid containing calcium sulfate crystals from process vessels. Additional sampling devices and procedures are discussed by A. N. Bauman and H. H. Roberts in "Automated Sampling and Analysis of Wet Process Phosphoric Acid Systems", Automation in Analytical Chemistry, Oct. 2, 1967, and P. K. Bhattacharjee and B. L. Winslow in "Experience with On-Line Sulfate Analyzer", 190th ACS Meeting Sept. 9-12, 1985. However, the disclosed devices require extensive maintenance to prevent component failure and plugging of sampling tubes and lines with precipitated solids. In addition, the pipe that suspends the filter in the reactor must be long enough to reach well below the level of the slurry contained in the vessel. In a large vessel, it is difficult to withdraw the pipe for changing the cloth on the filter or for other service. If the contents of the vessel are maintained at subatmospheric pressure, such as the contents of the crystallizer, it is difficult to service the filter without disrupting the vacuum in the vessel. The pipe is also subject to agitation-induced lateral forces that may damage it or the vessel agitator.
None of the above-identified references disclose flow-through sampling devices which are adapted to obtain clear samples from pressurized pipelines. A flow-through device would have the following significant advantages in comparison to conventional devices: (1) it would provide that the slurry contacting the filter is fresh and representative of slurry within the process, (2) it would provide a means for sweeping away the solids that are periodically backwashed off the sample filter, and (3) it would produce a "scouring" action due to the rapidly moving slurry, thereby preventing the deposition and accumulation of scale on the surfaces of the sampler in contact with the slurry. Sampling from a pressurized pipeline also produces a greater driving force for filtration.
It is therefore a principle objective of the present invention to provide a reliable, flow-through sampling device for obtaining filtrate samples from pressurized pipelines containing a slurry of phosphoric acid and calcium sulfate, which is adaptable for automatic sampling, and which is resistant to failure caused by scaling or plugging due to precipitation formation on the filter surface or in the sampling lines or values.